The present invention relates to the voltage measuring technique for sheathed power cable laid out on an urban district comprising a sheathing material where high voltage is applied.
The current power distribution system is connected with a great variety of the loads of the users having different peaks of power demands or the users having power requirements greatly differing with time, as well as with a great variety of generators including decentralized power sources. To verify whether users are supplied with electric power produced by these generators, electric power measuring apparatuses for power cable must be installed at various positions of the power distribution system.
Generally, the electric power measuring apparatus for power cable can be classified according to the application, for example, according to whether it is sheathed power cable or bare power cable (power cable with bare cable conductor), or according to the structure of the electric power measuring apparatus, according to whether it is of grounded or ungrounded type.
One of the prior art voltage measuring apparatuses for sheathed power cable of non-contact type is found in the Japanese Application Patent Laid-Open Publication No. Hei 05-26907. According to this Publication, a capacitance voltage divider for high voltage is used to divide the voltage applied to the parasitic capacitance to ground formed between the sheathing material of the sheathed power cable and the ground surface. The divided voltage is subjected to optical detection using a pockels device. FIG. 2 illustrates this description in terms of an equivalent circuit. In other words, the parasitic capacitance to ground Cs of about 1 pF and serial voltage-divided capacitance Cd are connected to the power cable through coupling capacitance Cc by the power cable sheath, thereby detecting the terminal voltage of the voltage-divided capacitance Cd using a pockels device.
Further, one of the prior art voltage measuring apparatus for bare power cable of contacted type is found in the Japanese Application Patent Laid-Open Publication No. Hei 01-58739.
In the prior art, measurement of the voltage applied to the cable conductor of the sheathed power cable installed in the urban district is likely to be affected by the resistance of surface leakage due to contamination of the sheathing material, rain or snow. Moreover, voltage requires a high measuring precision for operational purposes. This has made it difficult to meet such voltage measurement requirements.
In the art disclosed in Japanese Application Patent Laid-Open Publication No. Hei 01-26907, the operation voltage of the pockels device having the required sensitivity is as high as several hundred volts, so the value of voltage-divided capacitance Cd is reduced about 10 pF, with the result that the voltage divider is necessarily composed of a series circuit of small capacitance (three capacitors). Accordingly, multiple joints of high impedance as shown in a and b are formed, resulting in an increased sensitivity to leak resistor R11 on the surface of the voltage measuring apparatus enclosure generated with respect to the joints and leak resistor R12 on the surface of the power cable sheath. Thus, the leak current of on the order of Gxcexa9 cannot be ignored, and there is a limit in terms of principle when consideration is given to protect against the influence of the environment including rain, icy snow, dust, salt damage and others. In other words, this method is not insufficient to provide against possible deterioration of precision due to the environment.
The art disclosed in Japanese Application Patent Laid-Open Publication No. Hei 01-58739 relates to a voltage measuring apparatus for shaft power cable. This is not applicable to the measurement of voltages of sheathed power cables used in Japanese urban districts.
Furthermore, the prior arts explained above do not provide a jointing member capable of fixing the voltage measuring apparatus in position without damaging the power cable.
The object of the present invention is to improve measuring precision in an voltage measuring apparatus for sheathed power cable.
Another object is to provide a voltage measuring apparatus for sheathed power cable where the sheathed power cable can be protected.
One embodiment of the present invention is a voltage measuring apparatus for sheathed power cable comprising;
a first conductive member arranged to contact the sheathing material for sheathing a power cable conductor,
a second conductive member for forming a capacitance between the ground and itself,
a current voltage converting means for conversion into the voltage signal having a waveform proportional to the current signal flowing between the first conductive member and the second conductive member, and
a voltage value calculating means for calculating the voltage value applied to the power cable from the voltage signal converted by the current voltage converting means. Another embodiment is a voltage measuring apparatus for sheathed power cable comprising;
a first conductive member arranged below the sheathing material for sheathing a power cable conductor,
a second conductive member arranged below the first conductive member,
a current voltage converting means for conversion into the voltage signal having a waveform proportional to the current signal flowing between the first conductive member and the second conductive member, and
a voltage value calculating means for calculating the voltage value applied to the power cable from the voltage signal converted by the current voltage converting means and gain coefficient.
As described above, the current connected in series between the first and second conductive members and flowing these conductive members is changed into the voltage having the waveform proportional to this current, and the voltage value applied to the sheathed power cable is obtained from the waveform of the converted voltage. This reduces the influence of leak resistance, thereby ensuring a high precision of the voltage value applied to the cable conductor of the power cable.
When the voltage applied to the power cable is calculated from the voltage converted by this current voltage converting means, the root-mean-square value of the converted voltage is found out, and this root-mean-square value further is operated using the gain count thereby getting the voltage. This method is possible because the root-mean-square value is calculated by the gain count obtained from the relationship between the current proportional to the waveform of the voltage applied to the power cable conductor and the voltage applied to the power cable conductor, and the relationship with voltage proportional to that current waveform.
A large coupling capacitance can be ensured by the arrangement made to ensure that the first conductive member has a certain area. This substantially eliminates the voltage dividing point of small capacitance sensitive to leakage.
When the first conductive member is made of a member softer than the sheathing material or more elastic one, the voltage measuring apparatus for sheathed power cable can be installed in such a way that possible damages to the sheathed power cable and/or heating material can be reduced.
The first conductive member is preferred to be made of rubber, sponge or sponge consisting of rubber. Further, it can be verified by experiments that silicone is a suitable material constituting the first conductive member.
The power cable voltage measuring apparatus may be subjected to deterioration of measuring precision after a long-term use.
To solve this problem, the power cable voltage measuring apparatus according to the present invent includes an embodiment where a gain coefficient rewriting means is provided to rewrite the gain coefficient in the power cable voltage measuring apparatus in conformity to the installation environment.
Precision can be improved by this means for repeating rewriting.
The present invention also includes an embodiment comprising a gain count transmission means for sending from the outside the gain coefficient to be rewritten by the gain count rewriting means.
In the embodiment where a radio communications means is used as this transmission means, wired connection with the outside is completely eliminated with the result that there is no salt damage or leakage which would be caused by the use of wired connection.
Measuring precision may be reduced by reduction in potential on the surface of the power cable sheath due to salt damage or the like. To avoid the reduction in measuring precision, the present invention provides an embodiment comprising a means for making the voltage on the surface of the sheathing material almost the same as the voltage applied to the power cable conductor. One of the variations of this embodiment comprises a metallic pin for electric conduction with the power cable conductor and a ring-formed conductive rubber having an electric connection with this metallic pin.